All of the indicators that you mention make their age more than 5 years old, and based on the birth of Seth less than 130 years old.
... since Genesis 5:3 informs us that Adam was 130 years old when Seth was born, it is obvious that Adam and Eve could not have been in the Garden of Eden any longer than 130 years!
How Long Were Adam and Eve in the Garden of Eden? End of the 5th paragraph
The first 5 paragraphs of this link talk about how long they were in the Garden of Eden, and I think that is really going to be part of this answer no matter how you look at it.
Back to the indicators,
- naming animals
these things can be done by a 5 year old (give or take a year or more)
- falling in love
- being lonely (or needing a helper, as indicated as the reason for God to create Eve)
These things can happen in a young child or adolescent depending on your definition of falling in love, I would say that because God created one for the other that they could have instantly fallen in love, seeing that they literally completed each other.
My vote is that this happened in early adulthood when Adam was still young enough to till the earth.
I believe that these things happened through the stages of life (like growing up), much as they do for us, so it is entirely possible that He created Adam and Eve as Children or even took care of them as Babies until they were old enough to thrive in the garden.
I don't think that these things all happened in one day, I think it happened over an extended length of time where Adam and Eve developed skills and knowledge and other things they would need to survive on their own (when they left the garden)
Unfortunately, this is more "my opinion" than biblical fact.